cohesion model sjkr/powder

Purpose

Modelling material cohesion using a modified version of the simplified Johnson-Kendall-Roberts (SJKR) cohesion model.

Syntax

cohesion sjkr/powder [other model_type/model_name pairs as described here ] settings keyword values

zero or more keyword/value pairs may be appended after the keyword settings (after all models are specified)

enable_sjkr_yield values = 'on' or 'off'
   on = tangential force is set to zero if yield condition is satisfied
   off = tangential force is always applied
tangential_reduce values = 'on' or 'off'
  on = tangential model does not see normal force computed by this model
  off = tangential model does see normal force computed by this model

Associated material properties

Material interaction properties

cohesionEnergyDensity ( $k_0$ ): original cohesion energy density between the two materials [energy/length^3]
cohesionEnergyDensityVariation ( $k_{var}$ ): coefficient quantifying the change of cohesion energy density as a function of $\alpha$ [ $\cdot$ ]
cohesionYieldFactor ( $c_y$ ): maximum ratio of tangential to normal force [ $\cdot$ ]

Description

This model can be used as part of pair gran and fix wall/gran.

The only difference between this model and the SJKR is that the cohesion energy density is a function of the compression status of the material, quantified by the variable $\alpha$ (see fix powder/update), i.e.,

$k = k_0 \left(1 + \frac{r_i^2 a_i + r_j^2 a_j}{r_i^2 + r_j^2}(k_{var} - 1)\right)$

where $r_i$ and $r_j$ are the radii of particles $i$ and $j$ , and

$a_i = \frac{\alpha_i - \alpha_{min}}{1.0 - \alpha_{min}}.$

In order to access $\alpha$ , this model needs requires the fix powder/update command.

The optional keyword tangential_reduce specifies if the tangential force model should be affected by the additional normal force exerted by this model. If set to ‘off’ (which is default), then the tangential force model will take into account the extra normal force generated by this model; hence, the tangential force model will be able to transmit a larger amount of tangential force. If set to ‘on’, then the tangential model will not take the normal force from this model into account, typically leading to a lower value of tangential force (via the Coulomb friction limit).

If the optional keyword enable_sjkr_yield is set to ‘on’ then the tangential force is only active if the ratio of calculated tangential force to normal force from all previous models (normal, tangential and rolling) is lower than the yield limit $c_y$ . This allows to simulate failure behaviour with this model.

Restrictions

The cohesion model has been derived for the Hertzian style, it may note be appropriate for the Hookean styles.

It is not available for atom_style superquadric.

Coarse-graining information:

Using coarsegraining in combination with this command should lead to statistically equivalent dynamics and system state.

Default

tangential_reduce = ‘off’, enable_sjkr_yield = ‘off’